The production of magnet wire requires that a quality product be produced at a rapid rate in order to keep up with the demands of the marketplace. Competition in this area requires that such wire be produced economically, which is accomplished by selecting a rate and method of production in order to produce the maximum amount of wire at a minimum of cost, costs which include raw materials and process time.
A typical method of fabrication for such wire is a three-step process. In the first step, the "annealing step", bare wire of a desired diameter is drawn from a supply through an annealing oven in order to soften it and increase its flexibility as required in subsequent process steps.
In the second step, the "coating step", the bare wire exiting the annealing oven passes through a slip containing an "insulating enamel" typically a mixture comprised of polymers in organic solvents. The wire drawn through the slip then passes through a die having a passage of a dimension to allow only the wire and a layer of insulating enamel adhering to the wire to pass through to form a "coated wire". Any excess insulating enamel is returned to the slip.
The coated wire passes next to the third step, the "drying and curing" step. For this step a vertical oven is typically used, one maintained to have a temperature gradient across the path of the coated wire being drawn through it. At the bottom where the coated wire enters, the temperature is maintained to allow the gradual evaporation of the organic solvents in the insulating enamel. At the top end of the oven where the coated wire exits, a relatively higher temperature is maintained to insure the curing of the polymers in the insulating enamel. The wire having a layer of cured insulating enamel and exiting from the top of the oven comprises an "insulated wire" which may be wound onto a takeup spool.
The wire formed as noted in the above-outlined process sequence is rarely useful as the insulation layer formed by one coating step is too thin. Rather, the insulated wire is taken from the oven and redrawn through the coating step where a second layer of insulating enamel adheres to the wire, which is then passed through a second die which has a passage of a larger dimension than the first die, and drawn through the oven for drying and curing of the second layer. It should be noted that the second die functions in the same manner as the prior die, and differs only in that its passage is of a slightly larger dimension than that of the prior die. This allows for the thickness of the insulation on the wire to be increased by the second coating step.
This method of adding coatings to the wire may easily be repeated any desired number of times and requires only that successive dies have passages with progressively increasing dimensions to assure the formation of a plurality of progressively increasing insulation layers on the wire. This method of multiple coating is preferred as it is well known in the art that insulated wires formed with a plurality of insulating layers are more flexible and more resistant to cracking of insulation than wires having one thick layer of insulation.
The dies used in the coating step may be retained in a structure known as a die bar. Typically, the die bar is a rectangular bar having suitably formed holes within which individual dies may be secured. The die bar itself is positioned above the slip and below the oven, and acts to retain the dies in this location, and to simultaneously provide separation between the wires passing through the dies, assuring that they will maintain a generally parallel orientation as they pass between the slip and the oven entrance. Further retention of parallel orientation of wires is provided by two sets of sheaves, a first "feed sheave" set situated below the slip, die bar and oven which acts to receive bare wire from a feed spool and insulated wire returning from the oven exit to be recoated, and a second "return sheave" set of sheaves situated above the exit of the oven, which acts to receive insulated wire exiting the oven and return it to the lower sheave for subsequent recoating, or return it to a takeup spool where the fully insulated wire or "product wire" is wound. Generally, the spacing between the wires passing around the first and second set of sheaves is equal to the spacing of the wires passing through the die bar. This assures that the wire remains parallel during the second and third steps of the process which is frequently repeated. It is seen then, that the wire taken from a supply spool for coating may complete several successive "passes" of being processed, namely successive cycles of "coating" steps followed by drying and curing steps. Each pass of wire then receives an additional coat of enamel insulation, and the number of passes that each wire takes determines the number of coats of insulating enamel. Additionally, the path that the wire takes through the machine in the completion of each pass defines the "process path".
The production of enamel insulated wire usually entails the simultaneous production of several separate wires, each undergoing multiple passes through the process path. In this way, several separate wires may be simultaneously produced to effect a savings in production time and energy. This method requires that several die bars be used, where each die bar contains dies having passages of the same dimension, and the several die bars are serially positioned in the wire coating apparatus so that the wire progressively passes from the die bar having the smallest size dies to the die bar having the largest size dies during the multiple passes of the coating operation. The several die bars having such a placement assures the formation of progressively increasing insulation layers on each wire during the process.
This method of simultaneously producing several insulated wires by multiple passes through the process path, requires a "multi-wire multi-pass" operation where, at the start up of production, the wires be threaded through the complete process path for each pass. This operation may be performed manually by an operator who threads ends of the several feed wires from the annealing oven around sheaves in the first set of sheaves, then through the slip and then threads each wire through dies in a first die bar, then pulls the wires through the oven and threads them around sheaves in the second set of sheaves and return them to the first set of sheaves. This process continues until the wires have been passed through the process path for the desired number of passes and the requisite number of dies. Then the wires are threaded onto takeup spools, and once the wire is completely threaded, the production of magnet wire may begin.
This process is time consuming, difficult, and wasteful of material, as wire frequently breaks during the threading operation as a consequence of the prior annealing process which has softened the wire. Further, the wire used for threading the coating apparatus must be discarded as waste, as the portions which pass through the hot oven have remained in the oven for a time greater than the normal "residence time", namely, the normal length of time that any segment of the wire remains in the oven during normal processing. Wire which has been treated in excess of the normal residence time frequently suffers unacceptable oxidation on their surface due to the prolonged heat exposure encountered. Excess oxidation of a wire results in the formation of flakes on the outer surface which frequently fall off which does not provide a good surface for the adhesion of insulating enamel. Accordingly, a better method of threading wire along the process path of a wire coating apparatus is needed.
This problem was addressed in commonly assigned U.S. patent application titled "Die Bar Carrier", Ser. No. 867,717, filed May 27, 1986. There an apparatus is disclosed which is used to thread the wires in a multi-wire multi-pass wire coating process through the wire coating apparatus. A die bar carrier is used in conjunction with a pair of parallel transport cables which run along the process path, and a die bar. In use, the die bars containing a plurality of dies are threaded with the ends of bare wires passing from the annealer, and then the die bar carrier is placed underneath the die bars and is used to simultaneously engage the transport cables and support the die bars. The moving transport cables carry the threaded die bars and wires through the process path and automatically thread the wires onto the proper sheaves during its travel. The transport cables can be stopped at any point so as to allow the manual disengagement of bar carrier, and remove it from among the wires and from beneath the die bar in order to position the die bar in the wire coating apparatus. The die bar carrier could then be reinserted between the transport cables and the wires and beneath the remaining die bars, and reengaged. Then the transport cables could be restarted to complete another pass where the next die bar could be released. This process continues until all of the die bars have been positioned and wires have been threaded through the wire coating apparatus. This die bar carrier offers significant advantages over hand threading wires through a wire coating apparatus, but there remains a continuing need in the art for devices which may be utilized in threading a wire coating apparatus used in multi-wire, multi-pass coating process.